Daniel Read wrote in his response to Kenneth LeFebvre's letter:
In XP, the tests that the customer is involved in are the "Acceptance Tests" a.k.a. "Customer Tests", and the customer may be assisted by or represented by professional testers (with or without programming ability), perhaps aided by a an acceptance-testing tool like "Fitnesse".
If these tests are specified before implementation, then there are two levels of test-first happening: the acceptance tests done first by testers/programmers, representing scenarios specified by the customer, and code + unit tests done test-first by pairs of programmers.
For more on Fitnesse, please see http://fitnesse.org/FitNesse.WhatIsFitNesse.
Daniel Read Responds
Thank you, Keith. My knowledge of Extreme Programming definitely contains holes, and this distinction between the acceptance tests that the customer participates in and the unit tests for which the coders are responsible was one of them. After this bit of back and forth, it appears that the three of us (you, Ken, and myself) are in basic agreement.
Thanks also for the link to FitNesse. This wiki by Ward Cunningham was helpful to me also.